Luminaires of the type used for street lighting are usually provided with a receptacle accommodating a photoelectric controller on top of the housing. The receptacle is wired into the power module or into the line supply to the luminaire. When photoelectric control of the lighting is desired, a controller is plugged into the receptacle. Insertion of the controller into the receptacle is usually done with a downward pressure and a twisting motion causing it to lock in, whence the common appellation twistlock. The controller opens the line circuit in the daytime but closes it at night. When it is desired to control the luminaire in some other way, as from a central switching location, a dummy plug is inserted into the receptacle which shortcircuits appropriate terminals.
When a luminaire equipped with a photoelectric controller is first installed in the field, the controller is ordinarily oriented north (in the northern hemisphere) to insure that the photocell does not receive direct sunlight which could shorten its life. In special situations the controller may be oriented to avoid facing floodlights or other lighting. Orientation of the controller usually requires the use of tools on the part of the installer. With one well-known design of receptacle, it is accomplished by loosening screws in the top of the receptacle, turning the receptacle until an arrow (or the legend N) is aimed toward north, and then tightening the screws again.
General desiderata in receptacle design are ease of assembly into the luminaire at manufacture, convenience of installation or adjustment in the field, combined with minimum number of parts, low cost and reliability. Particularly desirable are features which reduce the burden of field installation because the installer may have to work high above the ground or under precarious conditions.